P1118
SubaruManifold Absolute Temperature Circuit Low Input
Medium SeverityThis code indicates that the Engine Control Module (ECM) is receiving a voltage signal from the Intake Air Temperature (IAT) sensor that is lower than expected. This low input usually means the sensor or its circuit has a problem, potentially causing the ECM to miscalculate fuel delivery and ignition timing based on incorrect air temperature readings.
⚠️ Before You Begin (Safety First)
Always ensure the engine is cool before working on any under-hood components to avoid burns. Disconnect the negative battery terminal before inspecting electrical circuits to prevent accidental short circuits or electrical shocks. Use proper jack stands if raising the vehicle.
Common Symptoms
- Check Engine Light (CEL) illuminated
- Rough idle
- Reduced fuel economy
- Poor engine performance (especially during cold starts)
- Increased emissions
Common Causes
- Faulty Intake Air Temperature (IAT) sensor
- Corroded or loose electrical connections at the IAT sensor or ECM
- Open or short in the IAT sensor wiring harness
- Damaged ECM (less common, but possible)
Frequently reported on: Subaru Forester (2000-2005), Subaru Impreza (1998-2004), Subaru Legacy (1999-2004), Subaru Outback (2000-2004)
Diagnostic Steps
- First, let's visually inspect the Intake Air Temperature (IAT) sensor. On most Subaru engines, you'll find it either integrated into the Mass Air Flow (MAF) sensor or as a standalone sensor located in the intake manifold or air intake tube. Look for any obvious physical damage or disconnected wires. This is our starting point for understanding what the ECM is 'seeing'.
- Next, carefully disconnect the electrical connector from the IAT sensor. Check the terminals inside the connector for any signs of corrosion, bending, or dirt. Clean them if necessary using electrical contact cleaner. A poor connection can mimic a faulty sensor, so we want to ensure the signal path is clear.
- With the connector disconnected, you'll want to test the IAT sensor's resistance. You'll need a multimeter for this. The resistance of the sensor changes with temperature. Typically, at room temperature, it should show a certain resistance (you'd consult a service manual for exact values, but generally a few thousand ohms). If it reads open circuit (infinite resistance) or very low resistance, the sensor itself is likely faulty. This test helps us understand if the sensor is providing a signal at all.
- Now, let's check the wiring itself. With the ignition off and the negative battery terminal disconnected for safety, you'll want to perform a continuity test between the IAT sensor connector and the ECM connector (refer to a wiring diagram for pin locations). Also, check for shorts to ground or power. A break or short in the wire would explain why the ECM is getting a 'low input' signal, as the voltage isn't making it back correctly.
- If the sensor and wiring check out, the next step would be to replace the IAT sensor. This is often the most common culprit. Install the new sensor, reconnect the electrical connector, and clear the P1118 code using an OBD-II scanner. Then, take the vehicle for a test drive to see if the code returns. If the sensor was the issue, the ECM should now be receiving the correct temperature readings.
👨🔧 When to Call a Professional
If you've checked the sensor and wiring and are still getting the code, or if you're uncomfortable using a multimeter to test electrical circuits, it's time to call a professional. Complex electrical diagnostics often require specialized tools and knowledge.
Commonly Asked Questions
What exactly does 'low input' mean for the IAT circuit?
When the ECM sees a 'low input' from the IAT sensor, it means the voltage signal coming from the sensor is lower than the ECM expects under normal operating conditions. A lower voltage signal usually correlates to a higher resistance in the sensor, which the ECM interprets as colder air. The problem arises when this signal is so low it's outside the sensor's plausible range, indicating a circuit issue rather than actual cold air.
Can I drive my Subaru with a P1118 code?
While you might be able to drive your Subaru, it's generally not recommended for extended periods. The ECM is receiving incorrect air temperature data, which affects how it calculates fuel mixture and ignition timing. This can lead to decreased fuel economy, rough engine operation, and potentially increased emissions. It's best to address it promptly to prevent further complications and ensure optimal engine performance.
How does the IAT sensor affect engine performance?
The IAT sensor is crucial because the density of air changes with temperature. Colder air is denser and contains more oxygen, requiring more fuel for proper combustion. Warmer air is less dense and needs less fuel. If the ECM gets a faulty low input (thinking the air is extremely cold when it isn't), it might unnecessarily enrich the fuel mixture, leading to a 'rich' condition and poor fuel economy. Conversely, if it were to receive a high input (thinking the air is very hot), it might lean out the mixture too much. In this 'low input' case, it typically means the ECM thinks the air is colder than it actually is.
Is the IAT sensor part of the MAF sensor on all Subarus?
That's a good question! On many newer Subaru models, the IAT sensor is indeed integrated into the Mass Air Flow (MAF) sensor assembly. However, on some older models, especially those from the late 90s to early 2000s, it might be a separate component located in the intake manifold or air intake ducting. Always consult your vehicle's service manual or a specific wiring diagram to pinpoint its exact location for your year and model.